Method for the production of (meth)acrylic acid esters

ABSTRACT

A process is described for preparing (meth)acrylic esters by reacting (meth)acrylic acid with C 4 -C 12 -alkanols in the presence of sulfuric acid or a mono-C 4 -C 12 -alkyl sulfate as catalyst, where the catalyst is regenerated by extraction with water from the reaction mixture and the aqueous catalyst solution is recycled back to the esterification, the concentration of unreacted alkanol in the reaction mixture to be extracted being not more than 5% by weight, based on the reaction mixture to be extracted.

The invention relates to a process for preparing esters of acrylic acidor methacrylic acid [(meth)acrylic acid].

(Meth)acrylic esters are generally prepared industrially by esterifying(meth)acrylic acid with alkanols in the presence of strong acids asesterification catalysts (eg. a mineral acid, such as sulfuric acid orphosphoric acid, alkanesulfonic acids or arylsulfonic acids). Processesof this type are disclosed, for example, by Kirk Othmer, “Encyclopediaof Chemical Technology”, Vol. 1, pp. 347-348. The content of catalystsin the esterification mixture can vary in an order of magnitude from atenth of a percent to several percent. When polybasic mineral acids areused as catalyst, the mineral acid is readily esterified by the alkanolpresent, forming the monoester, which is the actual esterificationcatalyst. The reaction mixture contains a relatively large amount ofthis monoester when the esterification is completed.

The acids used as catalysts and their esters which may be formed must beeliminated from the reaction mixture prior to further processing.Generally, this is achieved by extracting and neutralizing the reactionmixture with alkali metal hydroxide solution and alkaline earth metalhydroxide solution or carbonate solutions. This operation produceswastewaters whose disposal is complex and environmentally polluting. Ifsulfuric acid is used as catalyst, as mentioned, the monoester ofsulfuric acid with the alkanol in question is primarily formed. Thesalts of the sulfuric monoesters, in particular of the esters withhigher alkanols, are surface-active and on their disposal they wouldconsiderably impair the quality of the wastewaters from the process andwould cause a not insignificant loss of product of value. For economicand ecological reasons, recovery and recycling of the catalyst is thusdesirable.

The prior art includes a plurality of processes, all of which areburdened with considerable disadvantages, however.

EP-A-0 609 127 describes a process for preparing (meth)acrylic esters,in which the alcohol component is recovered by acid hydrolysis from thecorresponding sulfuric monoester which is formed from sulfuric acid andthe alcohol during the esterification. This process is complex,environmentally polluting and uneconomic.

CZ-B-179 808 describes a process for recovering mineral acids fromesterification mixtures by extracting the esterification mixture withwater, concentrating the aqueous phase by distillation and recycling theconcentrated aqueous catalyst solution thus obtained to theesterification reaction. This process is energy-consuming.

EP-A-0 618 187 ({circumflex over (=)} U.S. Pat. No. 5,386,052) describesa process for preparing (meth)acrylic esters, in which the catalyst isextracted with water and the extract, with or without concentration bydistillation, is recycled to the esterification reaction. However, it isparticularly emphasized here that sulfuric acid, owing to the poorextractability of the monoalkyl sulfate, is unsuitable as a catalyst,because the large amount of water which would be necessary for adequateextraction of the monoalkyl sulfate would adversely affect theesterification reaction. As catalyst, therefore, use is made ofalkylsulfonic or arylsulfonic acids (column 2, lines 55ff), which areconsiderably more expensive than sulfuric acid, however.

It is an object of the present invention to develop a technically simpleand economic process for preparing (meth)acrylic esters whichsuccessfully uses sulfuric acid as esterification catalyst and whichpermits the esterification catalyst (sulfuric acid or monoalkyl sulfate)to be separated off very simply and substantially completely from theresulting reaction mixture. In addition, the catalyst shall berecyclable back to the esterification directly, ie. without additionalconcentration by distillation, without affecting the esterification.

It has surprisingly been found that the catalyst can be extracted fromthe reaction mixture (esterification mixture) if the content of alkanolin said mixture is not more than 5% by weight.

We have found that this object is achieved by a process for preparing(meth)acrylic esters by reacting (meth)acrylic acid withC₄-C₁₂-alkanols, preferably C₄-C₁₀-alkanols, particularly preferablyC₄-C₈-alkanols, in the presence of sulfuric acid or a mono-C₄-C₁₂-alkylsulfate as catalyst, which comprises regenerating the catalyst byextraction with water from the reaction mixture and recycling theaqueous catalyst solution back to the esterification, the concentrationof unreacted alkanol in the reaction mixture to be extracted being notmore than 5% by weight, based on the reaction mixture to be extracted.

It has surprisingly been found that the alkanol content has a greatinfluence on the extractability of the monoalkyl sulfate, which isformed from sulfuric acid and alkanol and which acts as the actualesterification catalyst; surprisingly the catalyst can be extracted fromthe reaction mixture (esterification mixture) if the alkanol content inthe mixture is not more than 5% by weight (see Table 1). As a result,the catalyst can be extracted with small amounts of water, so that theextract can be directly recycled back to the esterification. Preferably,a reaction mixture whose alkanol content is ≦3% by weight, and inparticular ≦1% by weight, is extracted.

In order to achieve an alkanol content of not more than 5% by weight,preferably, a high degree of esterification is brought about, eg. bydistilling off the reaction water; and/or a suitable ratio of startingmaterials is selected. If the residual alkanol content is then stillmore than 5% by weight, the alkanol is distilled off in a conventionaldistillation apparatus (eg. column equipped with sieve trays, Raschigrings, ordered packings etc.). Surprisingly, despite the presence of thestrongly acid esterification catalyst, no acid-catalyzed side reactions,such as ether or olefin formation or addition of the alkanol to thedouble bond of the (meth)acrylate (Michael addition), may be observedhere to any significant extent.

The distillation is carried out in a customary manner; the distillationconditions depend on the type of alkanol used.

The alkanol is preferably distilled off down to a residual alkanolcontent in the reaction mixture which enables the catalyst (sulfuricacid) to be extracted with water without problems. In particular, theresidual alkanol content is ≦5% by weight, preferably ≦3% by weight,particularly preferably ≦1% by weight, based on the reaction mixture.

Preferably, the conditions for extracting the catalyst from theesterification mixture are selected in such a manner that the catalystconcentration (sulfuric acid and monoalkyl sulfate) in the aqueous phaseis at least 20% by weight, in particular at least 25% by weight, basedon the aqueous extract, and the degree of extraction is at least 70% byweight, in particular at least 80% by weight, based on the amount ofcatalyst in the reaction mixture. To achieve this, from about 5 to 20%by weight of water, in particular from about 8 to 15% by weight ofwater, based on the total weight of the esterification mixture, is usedfor the extraction. The resulting extract can be recycled, withoutconcentrating it, back to the esterification.

The extraction can be carried out in a manner known per se. Preferably,extraction is performed in counter-current, eg. in columns having noenergy input, pulsed columns, columns equipped with internals,mixer-settler apparatuses or in static mixers.

The extraction can be carried out at ambient temperature or above, butexpediently in the range from about 15 to 40° C.

The esterification is essentially carried out in a conventional manner,ie. by reacting (meth)acrylic acid with a C₄-C₁₂-alkanol in the presenceof a catalyst and at elevated temperature. The molar ratio ofalkanol:acrylic acid or methacrylic acid is generally 1:0.8-1.2.C₄-C₁₂-Alkanols are, for example, pentanol, hexanol, heptanol, octanol,2-ethylhexanol, nonanol, 2-propylheptanol, decanol, undecanol,dodecanol, and, preferably, butanols, in particular n-butanol. Thesulfuric acid concentration in the reaction mixture is generally from0.5 to 10% by weight, preferably from 1 to 5% by weight, based on thetotal batch.

As polymerization inhibitors, use is made of, for example,phenothiazine, hydroquinone, hydroquinone monomethyl ether, or mixturesof these with or without air (from 0.1 to 10 l/h×l) in an amount of 100to 5000 ppm, based on the reaction mixture. As an entrainer for removingwater from the reaction mixture, use can be made in the processaccording to the invention of saturated hydrocarbons (eg. cyclohexane)or aromatics (eg. toluene); preferably, the reaction is carried outwithout additional entrainer, however.

The reaction is generally carried out at from about 70 to 160° C.,preferably at from 90 to 130° C.

The reaction time is generally from about 1 to 10, preferably from 1 to6, hours.

The reaction can be carried out under atmospheric pressure, reducedpressure or superatmospheric pressure. Preferably, the pressure is setso that the water formed during the esterification distils off, forexample in the form of a mixture of water, C₄-C₁₂-alkanol and ester (theorganic components are fed back to the esterification in this case). Theesterification can be carried out continuously or batchwise, thecontinuous procedure being preferred.

The esterification is carried out in conventional apparatuses, eg. in anesterification unit of one or more heatable stirred reactors (cascade),which may be equipped with columns, condensers and phase-separationvessels. The reactor contents are mixed by stirring or other customaryand suitable measures.

If, after the extraction of the catalyst, a furtherextraction/neutralization of the residual acids (catalyst and(meth)acrylic acid) using an aqueous base is necessary, this can beperformed in a conventional extraction apparatus (see above), therequirement of base being low owing to the high degree of extraction ofthe catalyst and the extraction surprisingly proceeding without thephase-separation problems described in EP-A-0 566 074.

The ester is isolated in a conventional manner, in particular bydistillation, eg. by distillation in a sieve-tray column, from thereaction mixture which has been freed from catalyst and, if appropriate,residual carboxylic acid and low-boilers.

The Example below illustrates the invention, without restricting it.Percentages are by weight.

A stirred-tank cascade consisting of 3 stirred reactors, each of 1 lreaction volume, which are equipped with column, condenser andphase-separation vessel, was charged with 558 g of acrylic acid, 648 gof n-butanol, 16 g of sulfuric acid and 1 g of phenothiazine per hour.The reaction temperature in the reactors was 106° C., 118° C. and 123°C., respectively, and the pressure was 700 mbar. At the top of thecolumn, a mixture of water, n-butanol and n-butyl acrylate was produced,which separated into an aqueous and an organic phase, the organic phasebeing fed as reflux to the column.

The reaction efflux (1070 g/h) contained, according to analysis:

90.2% n-butyl acrylate

2.7% n-butanol

45 0.5% acrylic acid

2.2% monobutyl sulfate

Remainder: byproducts, polymers, oligomers, phenothiazine

Acrylic acid turnover: 99%, conversion 98%

The reaction efflux cooled to 25° C. was extracted in a mixer-settlerapparatus at approximately 25° C. with 90 g/h of water. The aqueousphase (97 g/h) contained 20.5% mono-n-butyl sulfate (85% recovery) and0.5% sulfuric acid.

This aqueous phase was recycled to the lower part of the distillationcolumn of the first reactor, with the addition of fresh sulfuric acidhaving been decreased to 1.3 g/h. The acrylic acid turnover was again99%, the conversion 98%.

The relationship between the degree of extraction of the catalyst andthe n-butanol content of the reaction mixture fed to the extraction wasdetermined by extracting esterification mixtures, which had beenprepared by esterifying acrylic acid with n-butanol in the presence ofsulfuric acid, and which had different n-butanol contents, once in aseparating funnel with 10% by weight of water at 25° C. The results ofthese experiments are shown in Table 1 and verify the importance of areaction procedure which keeps the content of unreacted alkanol below5%, preferably at from 0.1% to 3%.

TABLE 1 n-Butanol content  0.1%  2.5%  5.0% 10% Catalyst content  1.93% 1.88%  1.83%  1.75% Degree of extraction 89% 88% 71% 55% Residualcatalyst  0.22%  0.23%  0.53%  0.97% content

We claim:
 1. A process for preparing (meth)acrylic esters by reacting(meth)acrylic acid with a C₄-C₁₂-alkanol in the presence of sulfuricacid or a mono-C₄-C₁₂-alkyl sulfate as catalyst, which comprisesregenerating the catalyst by extraction with water from the reactionmixture, the concentration of unreacted alkanol in the reaction mixtureto be extracted being not more than 3% by weight, based on the reactionmixture to be extracted, and wherein the aqueous solution of thecatalyst regenerated by extraction with water from the reaction mixtureis recycled back to the esterification.
 2. A process as claimed in claim1, wherein the unreacted alkanol concentration is from 0.1 to 3% byweight.
 3. A process as claimed in claim 1, wherein the catalyst isextracted in such a manner that the degree of extraction is at least 70%by weight, based on the amount of catalyst in the reaction mixture.
 4. Aprocess as claimed in claim 1, wherein the catalyst is extracted in sucha manner that the catalyst concentration in the water phase is at least20% by weight, based on the aqueous extract.
 5. A process as claimed inclaim 1, wherein the extraction is performed at from 15 to 40° C.
 6. Aprocess as claimed in claim 1, wherein the extraction is carried out incounter-current or in a static mixer.
 7. A process as claimed in claim1, wherein the alkanol used is n-butanol or isobutanol.
 8. A process asclaimed in claim 1, wherein the reaction is performed at from 70 to 160°C.
 9. A process as claimed in claim 1, wherein the reaction time is from1 to 10 hours.
 10. A process as claimed in claim 3, wherein the catalystis extracted in such a manner that the degree of extraction is at least80% by weight, based on the amount of catalyst in the reaction mixture.11. A process as claimed in claim 4, wherein the catalyst is extractedin such a manner that the catalyst concentration in the water phase isat least 25% by weight, based on the aqueous extract.
 12. A process asclaimed in claim 8, wherein the reaction is performed at from 90 to 130°C.
 13. A process as claimed in claim 9, wherein the reaction time isfrom 1 to 6 hours.